Field of the Invention
The present invention relates generally to a stump grinding apparatus and, more specifically, to a stump grinding apparatus having a rotating wheel fitted with replaceable cutters.
Description of Related Art
A typical stump cutting or stump grinding apparatus includes a plurality of cutting teeth or tips mounted to a rotatable cutting wheel. The wheel is supported for rotation about a horizontal axis at relatively high speed at the end of a boom. When moved into position facing an unwanted tree stump, the boom together with the rotating wheel is swung side-to-side in a wide oscillating arc causing the individual cutting tips to grind and chip away the stump.
In the process of grinding away a tree stump below the surface of the surrounding grade, the cutting tips are driven into unknown perils for edged cutting tools including dirt and rocks and buried foreign objects and such. As would be expected, the cutting tips wear quickly due to the harsh operating environment, and are therefore typically held in serviceable tool holders, which are sometimes referred to in the trade as pockets or mounting blocks. Moreover, the tool holders also wear out and must be replaced often. Changing cutting tips and tool holders is not only expensive for operators, but also time consuming. Reducing the time and/or effort needed to service a stump cutting apparatus increases the overall benefits to an operator.
Despite the many recent advances in this art, many of which have been conceived and patented by the Applicant of this present invention, there exists need for improvements in at least three areas: serviceability, cutting efficiency and vibration reduction. The need for improved cutting efficiency is closely linked to the concept of driving power. Due to the characteristically low cutting efficiency of modern stump grinding systems, relatively large engines are needed to rotate the cutting wheel with sufficient torque and speed. Larger engines are expensive, heavy and consume large quantities of fuel. Stump grinding is an inherently rough, coarse operation. Tree stumps are often very hard, and/or non-uniform in hardness especially below the soil line. Rocks and other foreign objects are often embedded in the stump at or below the soil line as well. Cutting tips dull quickly. These factors contribute to a typically rough and jarring grinding experience. As a direct result, the machinery, components and fittings must all be made especially robust to withstand the typically difficult operating conditions. However, robust machinery, components and fittings are expensive and heavy, which in turn demand larger engines and more expensive/heavier equipment.
There have emerged many unique theories toward improving cutting efficiency. Some of these theories could not withstand the test of time, and are no longer practiced. Some theories, on the other hand, are based on insightful concepts but are hindered due to impractical elements in the initial designs. For example, U.S. Pat. No. 3,570,566 to McCreery describes a stump-grinding wheel having groups or clusters of side-cutting tips set in inward spirals. Each spiral cluster is composed of four or five spike-like teeth, each held in respective tool holders. The tool holders within each spiral cluster are all different sizes to position the cutting tip in a conical pattern. McCreery's concept of side-cutting tips set in inward spirals on the wheel body is insightful, however several inherent drawbacks have prevented this concept from gaining widespread commercial acceptable.
One drawback of the McCreey design relates to serviceability. The fact that the tool holders within each spiral group are all different sizes poses a serious burden to the operator, in that they must have on hand spares for each size tool holder which, in the case of McCreery's disclosed examples, will mean at least five different size tool holders. Having this many spares on hand is expensive and creates some inventory management burdens that many operators are loath to undertake. Furthermore, the numerous different size tool holders pose an opportunity installation errors. If the wrong size tool holder is installed in the wrong position on McCreery's wheel, it may go unnoticed until the wheel assembly is put into operation. A substantial decrease in grinding efficiency could be expected, together with accelerated wear and very likely accentuated vibration/chatter.
Another downside to McCreery's disclosed wheel concept is the failure to anticipate and remediate an inherent vibration/chatter caused by the positioning of the cutting tips in a conically-spiraling pattern. Because each cutting tip is laterally offset from the rotating plane of the wheel body, a bending moment is created in the wheel body in reaction to the cutting tip interacting with the stationary tree stump. The further the cutting tip is offset from the rotary plane of the wheel body, the greater its bending moment. It therefore follows that the bending moment generated at the innermost cutting tips is greater than the bending moments created any of the other cutting tips. In fact, within each spiral cluster, McCreery's cutting tips all produce different/unique bending moments. Consequently, the rapidly rotating wheel body being subjected to all these dissimilar forces begins to vibrate and chatter excessively, resulting in poor cutting performance and aggravation to the operator and accelerated wear and tear on all of the machinery.
There is therefore a need for a stump grinding system that has improved cutting performance using conically spiral clusters of cutting tips, but that produces less chatter/vibration and is relatively easy/inexpensive to maintain.